Various types of polyethylene are used in the art. Low density polyethylene (“LDPE”) can be prepared at high pressure using free radical initiators, and typically has a density in the range of 0.916-0.940 g/cm3. LDPE is also known as “branched” or “heterogeneously branched” polyethylene because of the relatively high level of long chain branches extending from the main polymer backbone. Polyethylene in the same density range, i.e., 0.916 to 0.940 g/cm3, which is linear and does not contain long chain branching, is also available; this “linear low density polyethylene” (“LLDPE”) can be produced with conventional Ziegler-Natta catalysts or with metallocene catalysts. Relatively higher density LDPE, typically in the range of 0.928 to 0.940 g/cm3, is sometimes referred to as medium density polyethylene (“MDPE”). HDPE has a density of greater than 0.940 g/cm3, and is generally prepared with Ziegler-Natta catalysts. Very low density polyethylene (“VLDPE”) can be produced by a number of different processes yielding polymers with different properties, but can be generally described as polyethylene having a density less than 0.916 g/cm3, typically 0.890 to 0.915 g/cm3, or 0.900 to 0.915 g/cm3.
Plastomers are ethylene/alpha-olefin copolymers with compositions and physical properties spanning the range between plastics and elastomers. Comonomer content typically ranges from 10 wt % to 30 wt % and density ranges from 0.910 to 0.860 g/cm3. Particularly useful plastomers are often ultra-low-density ethylene copolymers made using metallocene catalysts. The uniform comonomer insertion results in low-density plastomer exhibiting both plastic and elastomeric behavior. Compared to LLDPE, plastomers are lower in density, tensile strength, flexural modulus, hardness, and melting point. They exhibit higher elongation and toughness and are substantially higher in clarity, with very low haze values at lower densities.
Each product slate has unique properties and is used for specific applications. For some applications individual polymers do not possess the necessary combination of properties. For instance, LLDPE provides good toughness and other desirable properties but these properties decrease as the modulus (modulus is proportional to density for polyethylene) of the LLDPE increases. Generally, selecting optimum performance is a matter of trading off one property against another, for example, increasing modulus decreases toughness.
Individual polyolefins having certain characteristics are often blended together in the hope of combining the positive attributes of the individual components. Typically the result is a blend which displays an average of the individual properties of the individual resins. Blending has been used to form polymer compositions having altered properties, such as melt index and various processability characteristics. Blending has also been used to form polymer compositions having properties enhanced for particular end uses. For example, polymer blends have been used to form cast or extruded films with altered film properties, such as toughness, tear resistance, shrink properties, and other desired film characteristics. For example, U.S. Pat. No. 4,438,238 describes blends for extrusion processing, injection molding and films where a combination of two ethylene-α-olefin copolymers with different densities, intrinsic viscosities and number of short chain branching per 1,000 carbon atoms is attributed with such physical properties. U.S. Pat. No. 4,461,873 describes ethylene polymer blends of a high molecular weight ethylene polymer, preferably a copolymer, and a low molecular weight ethylene polymer, preferably an ethylene homopolymer, for improved film properties and environmental stress crack resistance useful in the manufacture of film or in blow molding techniques, the production of pipes and wire coating. EP 0 423 962 describes ethylene polymer compositions particularly suitable for gas pipes said to have improved environmental stress cracking resistance comprising two or more kinds of ethylene polymers different in average molecular weight, at least one of which is a high molecular weight ethylene polymer having an intrinsic viscosity of 4.5 to 10.0 dl/g in decalin at 135° C. and a density of 0.910 to 0.930 g/cm3 and another of which is a low molecular weight ethylene polymer having an intrinsic viscosity of 0.5 to 2.0 dl/g, as determined for the first polymer, and a density of 0.938 to 0.970 g/cm3.
U.S. Pat. No. 5,082,902 describes blends of linear polyethylene for injection and rotational molding said to have reduced crystallization times with improved impact strength and environmental stress crack resistance. The blends comprise: (a) a first polymer having a density of from 0.85 to 0.95 g/cm3 and an melt index of 1 to 200 g/10 min; and (b) a second polymer having a density of 0.015 to 0.15 g/cm3 greater than the density of the first polymer and an melt index differing by no more that 50% from the melt index of the first polymer. U.S. Pat. No. 5,306,775 describes polyethylene blends said to have a balance of properties for processing by any of the known thermoplastic processes, specifically including improved environmental stress crack resistance. These compositions have: (a) low molecular weight ethylene resins made using a chromium oxide based catalyst and having a density at least 0.955 g/cm3 and melt index (MI) between 25 and 400 g/10 min; and (b) high molecular weight ethylene copolymer resins with a density not higher than 0.955 g/cm3 and a high load melt index (HLMI) between 0.1 and 50 g/10 min.
U.S. Pat. No. 5,382,631 describes linear interpolymer polyethylene blends having narrow molecular weight distribution (Mw/Mn≦3) and/or composition distribution breadth index (CDBI) less than 50%, where the blends are generally free of fractions having higher molecular weight and lower average comonomer contents than other blend components. Improved properties for films, fibers, coatings, and molded articles are attributed to these blends. In one example, a first component is an ethylene-butene copolymer with a density of 0.9042 g/cm3, Mw/Mn of 2.3, and an MI of 4.0 dg/min and a second component is an HDPE with a density of 0.9552 g/cm3, Mw/Mn of 2.8, and an MI of 5.0 dg/min. The blend is said to have improved tear strength characteristics.
U.S. Pat. No. 6,362,270 describes thermoplastic compositions said to be especially suited to rotomolding applications comprising: (a) a majority component that may be an ethylene interpolymer having a density greater than 0.915 g/cm3 and preferably a melt index (MI) of from about 2 to 500 dg/min; and (b) an impact additive that may be an ethylene interpolymer having a density less than 0.915 g/cm3 and an MI preferably greater than 0.05 dg/min and less than 100 dg/min. Improved physical properties as ascribed to these compositions include improved impact strength and good environmental stress crack resistance.
Physical blends have problems of inadequate miscibility. Unless the components are selected for their compatibility they can phase separate or smaller components can migrate to the surface. Reactor blends, also called intimate blends (a composition comprising two or more polymers made in the same reactor or in a series of reactors) are often used to address these issues; however, finding catalyst systems that will operate under the same environments to produce different polymers has been a challenge.
Multiple catalyst systems have been used in the past to produce reactor blends of various polymers and other polymer compositions. Reactor blends and other one-pot polymer compositions are often regarded as superior to physical blends of similar polymers. For example U.S. Pat. No. 6,248,832 discloses a polymer composition produced in the presence of one or more stereospecific metallocene catalyst systems and at least one non-stereospecific metallocene catalyst system. The resultant polymer has advantageous properties over the physical blends disclosed in EP 0 527 589 and U.S. Pat. No. 5,539,056.
Thus, there has been interest in the art in developing multiple catalyst systems to produce new polymer compositions. For example, U.S. Pat. No. 6,147,180 discloses a thermoplastic elastomer composition comprising a branched olefin copolymer backbone and crystallizable side chains, wherein the copolymer has A) a Tg as measured by DSC less than or equal to 10° C.; B) a Tm greater than 80° C.; C) an elongation at break of greater than or equal to 300%; D) a tensile strength of greater than or equal to 1,500 psi (10.3 MPa) at 25° C.; and E) an elastic recovery of greater than or equal to 50%. The thermoplastic elastomer composition can be produced by A) polymerizing ethylene or propylene, optionally with one or more copolymerizable monomers, in a polymerization reaction under conditions sufficient to form a polymer having greater than 40% chain end-group unsaturation; and B) copolymerizing the product of A) with ethylene and one or more copolymerizable monomers so as to prepare said branched olefin copolymer. The two polymerization steps can be conducted sequentially or concurrently. Although the polymer exhibited good tensile properties and elastic recovery, the shear thinning was low. In addition U.S. Pat. No. 6,323,284 discloses a method to produce thermoplastic compositions (mixtures of crystalline and amorphous polyolefin copolymers) by copolymerizing alpha-olefins and alpha, omega dienes using two separate catalyst systems.
Likewise, others have experimented with multiple stage processes to produce new polymer compositions. For example, EP 0 366 411 discloses a graft polymer having an EPDM backbone with polypropylene grafted thereto at one or more of the diene monomer sites through the use of a two-step process using a different Ziegler-Natta catalyst system in each step. This graft polymer is stated to be useful for improving the impact properties in blended polypropylene compositions.
Although each of the polymers/blends described in the above references has interesting combinations of properties, there remains a need for new compositions that offer other new and different property balances tailored for a variety of end uses. In particular, it would be desirable to find a composition that contains cross products useful as compatibilizer compounds for interfacial interactions.
Other references of interest include: U.S. Patent Application Publication Nos. 2006/0281868; 2008/0027173; 2008/0033124; 2004/0054100; WO 2003/040201; U.S. Pat. Nos. 6,319,998; 6,284,833; 6,512,019; 7,365,136; 6,441,111; 6,806,316; 5,962,595; 5,516,848; 6,147,180; EP 0 527 589; and EP 0 749 992.